Spring 注解 @Resources, @Inject和@Autowired 的區別辨析

@TOC

本文簡述這三個spring應用里常用的注解區別。

@Resources

官方文檔里對@Resources的說明：

The @Resource annotation is part of the JSR-250 annotation collection and is packaged with Jakarta EE.

什么是JSR-250呢？訪問這個鏈接：https://jcp.org/en/jsr/detail?id=250

里面有很多PDF可以下載：

打開一看，其實就是Java支持的注解的文檔。這些文檔是最權威的：

文檔里介紹，@Resources對Bean的注入按照如下的優先級進行：

Match by Name

Match by Type

Match by Qualifier

Match by Name

我們來看看Match by name的例子。下面的代碼試圖通過Match by Name注入一個名稱為namedFile的Bean：

@RunWith(springJUnit4ClassRunner.class) @ContextConfiguration( loader=AnnotationConfigContextLoader.class, classes=ApplicationContextTestResourceNameType.class) public class FieldResourceInjectionIntegrationTest { @Resource(name="namedFile") private File defaultFile; @Test public void givenResourceAnnotation_WhenOnField_ThenDependencyValid(){ assertNotNull(defaultFile); assertEquals("namedFile.txt", defaultFile.getName()); } }

Bean的定義在如下代碼里：

@Configuration public class ApplicationContextTestResourceNameType { @Bean(name="namedFile") public File namedFile() { File namedFile = new File("namedFile.txt"); return namedFile; } }

運行時，申明Bean依賴處的@Resource的Name屬性和Bean定義處@Bean的Name屬性值一致，Match by Name測試通過。

Match by Type

將使用bean的消費者代碼里@Resource注解的name屬性去掉，使其變成下面這樣：

@Resource private File defaultFile;

測試仍然通過，是因為Match by Name的探測機制執行失敗后，進行下一輪Match by Type的探測，這一輪成功了。

Match by Qualifier

定義兩個Bean：

@Configuration public class ApplicationContextTestResourceQualifier { @Bean(name="defaultFile") public File defaultFile() { File defaultFile = new File("defaultFile.txt"); return defaultFile; } @Bean(name="namedFile") public File namedFile() { File namedFile = new File("namedFile.txt"); return namedFile; } }

測試代碼：

@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration( loader=AnnotationConfigContextLoader.class, classes=ApplicationContextTestResourceQualifier.class) public class QualifierResourceInjectionIntegrationTest { @Resource private File dependency1; @Resource private File dependency2; @Test public void givenResourceAnnotation_WhenField_ThenDependency1Valid(){ assertNotNull(dependency1); assertEquals("defaultFile.txt", dependency1.getName()); } @Test public void givenResourceQualifier_WhenField_ThenDependency2Valid(){ assertNotNull(dependency2); assertEquals("namedFile.txt", dependency2.getName()); } }

這一次執行失敗，遇到異常org.springframework.beans.factory.NoUniqueBeanDefinitionException.

原因是因為我們的測試代碼里，沒有指定注入Bean的名稱，因此Spring的Match by Name探測失敗，進行Match by Type時，探測到兩個類型一樣的Bean，Spring框架不知道注入哪一個，所以就報異常了。

避免這個異常也很容易，使用@Qualifier.代碼如下：

@Resource @Qualifier("defaultFile") private File dependency1; @Resource @Qualifier("namedFile") private File dependency2;

@Inject

這個注解定義在JSR-330里，文檔鏈接：

https://jcp.org/en/jsr/detail?id=330

注入的優先級：

Match by Type

Match by Qualifier

Match by Name

Match by Type

注意@Inject注入的最高優先級方式為Match by Type，而非@Resource的Match by Name.

任意定義一個待注入的Component：

@Component public class ArbitraryDependency { private final String label = "Arbitrary Dependency"; public String toString() { return label; } }

使用@Inject注入：

@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration( loader=AnnotationConfigContextLoader.class, classes=ApplicationContextTestInjectType.class) public class FieldInjectIntegrationTest { @Inject private ArbitraryDependency fieldInjectDependency; @Test public void givenInjectAnnotation_WhenOnField_ThenValidDependency(){ assertNotNull(fieldInjectDependency); assertEquals("Arbitrary Dependency", fieldInjectDependency.toString()); } }

Match by Qualifier

定義一個新的待注入組件：

public class AnotherArbitraryDependency extends ArbitraryDependency { private final String label = "Another Arbitrary Dependency"; public String toString() { return label; } }

測試代碼：

@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration( loader=AnnotationConfigContextLoader.class, classes=ApplicationContextTestInjectQualifier.class) public class FieldQualifierInjectIntegrationTest { @Inject private ArbitraryDependency defaultDependency; @Inject private ArbitraryDependency namedDependency; @Test public void givenInjectQualifier_WhenOnField_ThenDefaultFileValid(){ assertNotNull(defaultDependency); assertEquals("Arbitrary Dependency", defaultDependency.toString()); } @Test public void givenInjectQualifier_WhenOnField_ThenNamedFileValid(){ assertNotNull(defaultDependency); assertEquals("Another Arbitrary Dependency", namedDependency.toString()); } }

和之前@Resource的第一次試圖通過Match by Type注入一樣失敗，遇到異常：NoUniqueBeanDefinitionException

利用@Qualifier避免這個異常：

@Inject @Qualifier("defaultFile") private ArbitraryDependency defaultDependency; @Inject @Qualifier("namedFile") private ArbitraryDependency namedDependency;

Match by Name

public class YetAnotherArbitraryDependency extends ArbitraryDependency { private final String label = "Yet Another Arbitrary Dependency"; public String toString() { return label; } }

消費者代碼：

@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration( loader=AnnotationConfigContextLoader.class, classes=ApplicationContextTestInjectName.class) public class FieldByNameInjectIntegrationTest { @Inject @Named("yetAnotherFieldInjectDependency") private ArbitraryDependency yetAnotherFieldInjectDependency; @Test public void givenInjectQualifier_WhenSetOnField_ThenDependencyValid(){ assertNotNull(yetAnotherFieldInjectDependency); assertEquals("Yet Another Arbitrary Dependency", yetAnotherFieldInjectDependency.toString()); } }

application context代碼：

@Configuration public class ApplicationContextTestInjectName { @Bean public ArbitraryDependency yetAnotherFieldInjectDependency() { ArbitraryDependency yetAnotherFieldInjectDependency = new YetAnotherArbitraryDependency(); return yetAnotherFieldInjectDependency; } }

測試通過

@Autowired

這個注解和@Inject的用法一致，唯一區別就是@Autowired 屬于Spring框架提供的注解。例子略。

Java Spring

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版權聲明：本文內容由網絡用戶投稿，版權歸原作者所有，本站不擁有其著作權，亦不承擔相應法律責任。如果您發現本站中有涉嫌抄襲或描述失實的內容，請聯系我們jiasou666@gmail.com 處理，核實后本網站將在24小時內刪除侵權內容。